Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers comprising porous, electrically conductive sheet material and an electrocatalyst disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
In typical fuel cells, the MEA is disposed between two electrically conductive separator or fluid flow field plates. Fluid flow field plates have at least one flow passage formed therein to direct the fuel and oxidant to the respective electrode layers, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide support for the electrodes.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is typically held together in its assembled state by tie rods and end plates.
The stack typically includes manifolds and inlet ports for directing the fuel and the oxidant to the anode and cathode flow field passages respectively. The stack also usually includes a manifold and inlet port for directing a coolant fluid, typically water, to interior passages within the stack to absorb heat generated by the exothermic reaction in the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack.
In conventional fuel cell designs, such as, for example, the fuel cells described and illustrated in U.S. Pat. Nos. 3,134,697, 3,297,490, 4,057,479, 4,214,969 and 4,478,917, the plates which make up each conventional fuel cell assembly are compressed and maintained in their assembled states by tie rods. The tie rods extend through holes formed in the peripheral edge portion of the stack end plates and have associated nuts or other fastening means assembling the tie rods to the stack assembly and compressing the end plates of the fuel cell stack assembly toward each other. Typically the tie rods are external, that is, they do not extend through the fuel cell separator or flow field plates. One reason for employing a peripheral edge location for the tie rods in conventional designs is to avoid the introduction of openings in the central, electrochemically active portion of the fuel cells.
The peripheral edge location of the tie rods in conventional fuel cell designs has inherent disadvantages. It requires that the thickness of the end plates be substantial in order to evenly transmit the compressive force across the entire area of the plate. Also, the peripheral location of the tie rods can induce deflection of the end plates over time if they are not of sufficient thickness. Inadequate compressive forces can compromise the seals associated with the manifolds and flow fields in the central regions of the interior plates, and also compromise the electrical contact required across the surfaces of the plates and membrane electrode assemblies to provide the serial electrical connection among the fuel cells which make up the stack. However, end plates of substantial thickness contribute significantly to the overall weight and volume of the fuel cell stack, which is particularly undesirable in motive fuel cell applications. Also, when external tie rods are employed, each of the end plates must be greater in area than the stacked fuel cell assemblies. The amount by which the end plates protrude beyond the fuel cell assemblies depends on the thickness of the tie rods, and more importantly on the diameter of the washers, nuts and any springs threaded on the ends of tie rods, since preferably these components should not overhang the edges of end plate. Thus the use of external tie rods can increase stack volume significantly.
Various designs in which one or more rigid compression bars extend across each end plate, the bars being connected (typically via external tie rods and fasteners) to corresponding bars at the opposite end plate have been employed in an effort to reduce the end plate thickness and weight, and to distribute compressive forces more evenly. Such a design is described and illustrated in U.S. Pat. No. 5,486,430, which is incorporated herein by reference in its entirety.
A compact fuel cell stack design incorporating internal tie rods which extend between the end plates through openings in the fuel cell plates and membrane electrode assemblies has been reported in U.S. Pat. No. 5,484,666, which is incorporated herein by reference in its entirety.
The fuel cell stack compression mechanisms described above typically utilize springs, hydraulic or pneumatic pistons, pressure pads or other resilient compressive means which cooperate with the tie rods, which are generally substantially rigid, and end plates to urge the two end plates towards each other to compress the fuel cell stack.
Tie rods typically add significantly to the weight of the stack and are difficult to accommodate without increasing the stack volume. The associated fasteners add to the number of different parts required to assemble a fuel cell stack.
The present invention provides a simple, compact and light-weight compression mechanism for a fuel cell stack.